I’m assuming you mean milk kefir?
I make my own water kefir and drink about 1 quart a day. It is the only thing that made my constipation completely vanish (aside from taking over 1000mg of magnesium daily). I was thinking just today how glad I am not to have to buy Pro-Activ or similar sugary yoghurts just to keep my digestion moving. I think the amount you can tolerate is very individual, and Able is right about starting slow to see how you react to it. In my case, I have very sluggish digestion, so it has worked wonders. I think for some people the amount I consume would probably cause diarrhea.